Target and process for its production, and method for forming a film having a high refractive index

ABSTRACT

A sputtering target comprising a substrate and a target material formed on the substrate, wherein the target material comprises a metal oxide of the chemical formula MO x  as the main component, wherein MO x  is a metal oxide which is deficient in oxygen as compared with the stoichiometric composition, and M is at least one metal selected from the group consisting of Ti, Nb, Ta, Mo, W Zr and Hf, a process for its production, and a method for forming a film having a high refractive index.

TECHNICAL FIELD

[0001] The present invention relates to a target to be used for forminga transparent thin oxide film having a high refractive index by directcurrent (DC) sputtering, and a process for its production, and a methodfor forming a film having a high refractive index by using such atarget.

BACKGROUND ART

[0002] Optical applications of thin oxide films start from single layertype heat reflecting glasses and antireflection films and extend tovarious fields including, for example, multi-layer type antireflectioncoatings, reflection enhancing coatings, interference filters andpolarizing films, which are designed to permit lights having certainspecific wavelengths to reflect or pass selectively therethrough.Further, a study has been made to insert a transparent electroconductivefilm or a film of e.g. metal or electroconductive ceramics havingvarious functions such as electroconductivity and heat reflectionproperties as a part of a multi-layer film to obtain a multi-layer filmhaving a function such as an antistatic, heat reflecting orelectromagnetic wave shielding function provided.

[0003] The spectral characteristics of a multi-layer film are opticallydesigned by using refractive indexes n and thicknesses of the respectivelayers, as parameters, and it is common to employ a combination of ahigh refractive index film and a low refractive index film. To realizeexcellent optical properties, the larger the difference in therefractive index between the high refractive index film and the lowrefractive index film, the better. As such a high refractive index film,titanium dioxide (n=2.4) cerium dioxide (n=2.3), zirconium dioxide(n=2.2), niobium pentoxide (n=2.1), tantalum pentoxide (n=2.1) ortungsten trioxide (n=2.0) is, for example, known. Further, as a lowrefractive index film, silicon dioxide (n=1.46) or magnesium fluoride(n=1.38) is, for example, known.

[0004] Such films can be formed, for example, by a vacuum vapordeposition method or a coating method. However, by such a film-formingmethod, it is difficult to form a uniform film over a substrate having alarge area, and when a substrate having a large area, such as a glassfor buildings or automobiles, CRT, or a flat display, is required,sputtering is used in many cases. Among various sputtering methods, DCsputtering utilizing direct current discharge is most suitable forforming a film over a large area.

[0005] When a high refractive index film is to be formed by DCsputtering, it is common at present to employ so-called reactivesputtering wherein a metallic target having electroconductivity issubjected to sputtering in an atmosphere containing oxygen. However,there has been a problem that the film-forming speed of a thin filmobtainable by this method is very slow, whereby the productivity ispoor, and the cost tends to be high.

[0006] To solve such a problem, it has been proposed to use an oxideceramic (sintered body) as a target. However, oxide ceramic usually hasno electroconductivity, whereby DC sputtering has been difficult.

[0007] Further, recently, a sputtering target is required to have acomplex shape, and a highly efficient planer target having the targetthickness partially changed, is required. By a method for obtaining asintered body by a common sintering method, it is difficult to produce atarget having a complex structure or various shapes, and such a targetis prepared by a long process including steps of mixing startingmaterials, sintering, processing and bonding, whereby substantial jigsare required for its production.

[0008] In sputtering over a glass sheet with a large area for buildings,the film-forming speed is increased by applying a high power forsputtering to increase the productivity, whereby cooling of the targettends to limit the film-forming speed, and further troubles such ascracking of the target, peeling, etc, are likely to occur.

[0009] A new magnetron type rotary cathode is known wherein suchdrawbacks have been overcome (JP-A-58-500174). This is of a type whereina magnetic field generating means is provided inside of a cylindricaltarget, and sputtering is carried out while rotating the target andcooling the target from inside. By the use of such a cylindrical target,a large power per unit area can be applied as compared with a planertype target, whereby film formation at a high speed is said to bepossible.

[0010] Preparation of a target material on a cylindrical target holderhas heretofore been commonly carried out when the target material is ametal or alloy. In the case of a metal target, multi-layer film coatingsof e.g. its oxide, nitride, carbide, etc. are formed in varioussputtering atmospheres. However, it has had drawbacks that the coatingfilms are likely to be damaged by different types of atmospheres,whereby films having desired compositions can hardly be obtainable, and,in a case of a low melting metal target, the target is likely to undergomelting when the power applied is excessive. Under these circumstances,a ceramic target material has been desired. A method has been proposedin which a ceramic sintered body is formed into a cylindrical shape andbonded to a substrate by means of indium metal. However, the method isdifficult and costly.

[0011] JP-A-60-181270 proposes a process for producing a ceramicsputtering target by spraying. However, the process has had problemsthat the sprayed coating can not be made sufficiently thick, as thedifference in thermal expansion between the ceramics and the substratemetal is large, and the adhesion tends to deteriorate by thermal shockduring its use, thus leading to peeling.

[0012] JP-A-62-161945 proposes a process for producing anon-electroconductive ceramic sputtering target made of various oxidesby water plasma spraying. This target is a target for radio frequency(RF) sputtering, and the target itself is an insulating material.Further, this target has had drawbacks that, unless some measures suchan undercoating is taken, it is likely to undergo cracking or peeling asthe temperature rises during sputtering, whereby film formation under astabilized condition tends to be difficult. Further, there has been adrawback such that the film forming speed is very slow.

[0013] It is an object of the present invention to provide anelectroconductive sputtering target which can be formed into any desiredshape and which is capable of forming a high refractive index film at ahigh speed by DC sputtering, a process for its production, and a methodfor forming a high refractive index film using such a target.

DISCLOSURE OF THE INVENTION

[0014] The present invention provides a sputtering target comprising asubstrate and a target material formed on the substrate, wherein thetarget material comprises a metal oxide of the chemical formula MO_(x)as the main component, wherein MO_(x) is a metal oxide which isdeficient in oxygen as compared with the stoichiometric composition, andM is at least one metal selected from the group consisting of Ti, Nb,Ta, Mo, W. Zr and Hf.

[0015] The target of the present invention has electroconductivity andthus is useful for DC sputtering, whereby a uniform, transparent highrefractive index film can be formed at a high speed over a large area.The target of the present invention is useful also for RF sputtering.

[0016] In a case where M in MO_(x) of the target of the presentinvention is Nb and/or Ta, x is preferably within a range of 2<x<2.5.This means that if x is 2.5, the target is electrically insulating, asit is in a completely oxidized state, whereby DC sputtering is notfeasible, such been undesirable. On the other hand, if x is 2 or lower,such an oxide is chemically instable and, as such, is not desirable as atarget. When NbO_(x) is used, a high film-forming speed can be realized,and when TaO_(x) is used, it is possible to form a film having a highcorrosion resistance and high scratch resistance.

[0017] For the same reason as mentioned above, when M in MO_(X) of thetarget of the present invention is Mo and/or W, x is preferably within arange of 2<x<3, and when M in MO_(X) in the target of the presentinvention is at least one metal selected from the group consisting ofTi, Zr and Hf, x is preferably within a range of 1<x<2. Especially whenTiO_(x) is used, it is possible to realize formation of a film having avery high refractive index.

[0018] The target of the present invention has electrical conductivityand thus is useful for film formation by means of DC sputtering, wherebya uniform, transparent high refractive index film can be formed at ahigh speed over a large area. The resistivity at room temperature of thetarget of the present invention is preferably at most 10 Ωcm, morepreferably at most 1 Ωcm, so that discharge during sputtering can becarried out under a stabilized condition. If the resistivity exceeds 10Ωcm, discharge tends to be hardly stabilized.

[0019] For the target of the present invention, a composite oxide MO_(x)employing two or more metals M, may be used so that the target will havethe above mentioned characteristics simultaneously.

[0020] With the target of the present invention, the properties of thefilm such as the refractive index, and mechanical and chemicalproperties, can be changed while maintaining the high speed filmformation, by adding an oxide of a metal other than metal M in MO_(x),as an additive. As such a metal oxide, an oxide of at least one metalselected from the group consisting of Cr, Ce, Y, Si, Al and B, may bementioned. For example, Cr is capable of imparting corrosion resistance,and Ce is capable of imparting ultraviolet ray-shielding properties.

[0021] The target of the present invention can be prepared, for example,as follows.

[0022] In a case of a NbO_(x) target, a Nb₂O₅ powder is subjected tohot-pressing (high temperature and high pressure pressing) for sinteringto obtain a target of the present invention. In such a case, theparticle size of the powder is preferably from 0.05 to 40 μm. It isimportant that the atmosphere for the hot-pressing is a non-oxidizingatmosphere, and it is preferred to use argon or nitrogen, since it isthereby easy to adjust the oxygen content in the target. It is alsopossible to add hydrogen. The hot-pressing conditions are notparticularly limited, but the temperature is preferably from 800 to1,400° C., and the pressure is preferably from 50 to 100 kg/cm².

[0023] The present invention also provides a process for producing asputtering target, which comprises forming an undercoat made of a metalor alloy on a substrate, and forming a ceramic layer as a targetmaterial on the undercoat, wherein the ceramic layer as a targetmaterial (hereinafter referred to simply as the ceramic layer) is formedby plasma spraying wherein a ceramic powder for spraying (hereinafterreferred to simply as the ceramic powder) which is made in a semi-moltenstate in a high temperature plasma gas in a reducing atmosphere, istransported and deposited onto the undercoat by the plasma gas, and, asthe target material, a target material comprising a metal oxide of thechemical formula MO_(x) as the main component, is used, wherein MO_(x)is a metal oxide which is deficient in oxygen as compared with thestoichiometric composition, and M is at least one metal selected fromthe group consisting of Ti, Nb, Ta, Mo, W, Zr and Hf.

[0024] In the present invention, the ceramic powder is made in asemi-molten state by means of a plasma spraying apparatus and depositedon a substrate, so that a ceramic layer for a sputtering target isdirectly formed.

[0025] Accordingly, the process does not require a molding step, asintering step, a processing step to form a complex structure or shape,or a bonding step. In a case of a complicated compound which is notreadily available in the form of a ceramic powder, such a compound maybe chemically synthesized or may be prepared by using a solid phasereaction. The ceramic powder may be a pulverized or granulated, andfurther classified, so that it is adjusted to have a readily flowableparticle size suitable for spraying.

[0026] The ceramic powder to be used in the present invention can beprepared by the following method. Namely, a TiO₂ powder having anaverage particle size of at most 10 μm and a powder of a metal oxideother than TiO₂ having an average particle size of at most 10 μm, areweighed in predetermined amounts and mixed in a wet system for at least3 hours in a ball mill using a binder such as polyvinyl alcohol (PVA)and water as a dispersing medium, to obtain a slurry, which is thendried by a spray drier to obtain a powder having a particle size of from10 to 100 μm, preferably from 20 to 100 μm.

[0027] In another method, ethanol is used as the above mentioneddispersing medium, and a Nb₂O₅ powder and a TiO₂ powder are mixedtogether with ethanol in a wet system for at least one hour by means ofa ball mill in the same manner as described above, and the mixture isdried by an evaporator and then calcined in an inert atmosphere at atemperature of from 1,000 to 1,200° C., followed by classification toobtain a powder having a particle size of from 10 to 100 μm, preferablyfrom 20 to 100 μm. The composition of this powder is reduced bycalcination, but is further reduced during the subsequent plasmaspraying in a reducing atmosphere.

[0028] If the particle size exceeds 100 μm, such a ceramic powder tendsto be hardly made in a semi-molten state in a high temperature plasmagas, and if it is smaller than 10 μm, such a powder is likely to bedispersed in the high temperature plasma gas and thus tends to be hardlydeposited on the substrate.

[0029] For the substrate, various metals or alloys, such as stainlesssteel, copper or titanium, may be used. Prior to the plasma spraying ofa ceramic powder for the target material, it is preferred to roughen thesurface of the substrate, for example, by sand blasting by means ofabrasive grains made of Al₂O₃ or SiC in order to improve the adhesion.Otherwise, it is also preferred to process such a substrate surface toform a V-groove, followed by sand blasting by means of abrasive grainsmade of Al₂O₃ or SiC, in order to improve the adhesion.

[0030] After roughening the substrate surface, an undercoat made of ametal or alloy may be formed in order to reduce the difference in thethermal expansion between the target material to be sprayed and thesubstrate and to improve the adhesion so as to be durable againstpeeling by mechanical and thermal impacts.

[0031] As such an undercoat, a layer (hereinafter referred to as layerA) having a thermal expansion coefficient intermediate between thesubstrate and the target material, and/or a layer (hereinafter referredto as layer B) having a thermal expansion coefficient close to thetarget material, may be used. It is particularly effective to form bothlayers to have a structure of the substrate/layer A/layer B/ceramiclayer. It is preferred to form the undercoat also by plasma spraying.

[0032] Even when the undercoat is made solely by layer A or layer B, theadhesive force of the ceramic layer to the substrate can be improved,since the metal or alloy is not brittle and has high elasticity. Thethermal expansion coefficient of layer B is most suitably within a rangeof ±2×10⁻⁶/° C. of the thermal expansion coefficient of the ceramiclayer.

[0033] As the material for the undercoat, an electroconductive powder ofe.g. Mo, Ti, Ni, Nb, Ta, W, Ni—Al, Ni—Cr, Ni—Cr—Al, Ni—Cr—Al—Y orNi—Co—Cr—Al—Y may be employed. The thickness of the undercoat ispreferably from 30 to 100 μm.

[0034] It is necessary to change the material for the undercoatdepending upon the thermal expansion coefficient of the ceramic layer.The thermal expansion coefficient of e.g. copper or stainless steelwhich is useful as a substrate, is from 17×10⁻⁶ to 18×10⁻⁶/° C., and thethermal expansion coefficient of titanium is 8.8×10⁻⁶/° C.

[0035] For example, for the ceramic layer (the thermal expansioncoefficient: 6×10⁻⁶ to 9×10⁻⁶/° C.) in the present invention, thepreferred thermal expansion coefficient of the undercoat layer A is from12×10⁻⁶ to 15×10⁻⁶/° C., and as such a material, Ni, Ni—Al, Ni—Cr,Ni—Cr—Al, Ni—Cr—Al—Y or Ni—Co—Cr—Al—Y, may, for example, be mentioned.

[0036] Further, the preferred thermal expansion coefficient of theundercoat layer B is from 4×10⁻⁶ to 11×10⁻⁶/° C., and as such amaterial, Mo, Nb, Ta, W or Ti, may, for example, be mentioned.

[0037] Further, the adhesion can be further improved by providing anundercoat layer having the composition gradually changed from a materialhaving a thermal expansion coefficient close to the target material to amaterial having a thermal expansion coefficient close to the substrate,selected among such undercoat materials. Further, when the substrate ismade of titanium, the undercoat may be made solely of layer B, since thethermal expansion coefficient is close thereto.

[0038] On such an undercoat, a ceramic powder which is made in asemi-molten state in a high temperature plasma gas, preferably a hightemperature plasma gas such as Ar or Ar+H₂, in a reducing atmosphere, istransported and deposited onto the undercoat by such a gas, to form aceramic layer which serves as a target material. In this manner, theoxide ceramic powder is reduced, and a ceramic layer comprising MO_(x)as the main component, is obtained.

[0039] By forming the undercoat, the difference in thermal expansionbetween the ceramic layer and the substrate can be reduced, whereby aceramic layer which is free from peeling even with a thickness as thickas from 2 to 10 mm, can be formed.

[0040] Also at the time of forming the undercoat, it is preferred toform it by plasma spraying in a high temperature plasma gas, preferablyin a high temperature plasma gas in a reducing atmosphere, for the samereason as described above.

[0041] Further, as the plasma spraying method, water plasma spraying ismore effective. This water plasma spraying is a method wherein a highpressure water stream supplied to a torch firstly forms a cylindricaleddy current at the cylindrical section, and in this state, a voltage isapplied between a carbon cathode and an iron rotary anode to let directcurrent arcs form to evaporate and recompose water at the inner surfaceof the eddy current to form a plasma state thereby to generate plasmaarcs continuously, and such plasma arcs are constricted by the revolvingcylindrical water current to increase the energy density and eject astabilized high temperature high speed plasma jet flame from a nozzle byrapid thermal expansion of the plasma. This spraying method provides ahigh energy density as compared with a gas plasma density, and a largeamount of the starting material powder can thereby be sprayed all atonce, whereby the target forming speed is high, and the economicalefficiency is high. Further, it is thereby possible to readily form athick film.

[0042] However, as compared with plasma spraying by a reducing gas, thereducing power is weak. Therefore, to obtain a state of MO_(x), it isbetter to employ a material which is reduced at the stage of thestarting material powder.

[0043] The present invention further provides a method for forming ahigh refractive index film employing the above described target.

[0044] A uniform transparent film can be formed at a high speed, whensputtering is carried out by using the target of the present inventionin an argon atmosphere or in a mixed atmosphere of argon and smallamount of O₂ under a pressure of from 1×10⁻³ to 1×10⁻² Torr. In a casewhere a metal target is employed, a hysteresis phenomenon occurs whichis a non-continuous change in the film forming speed or the dischargecurrent or voltage due to change of the oxygen partial pressure.However, when the target of the present invention is employed, such ahysteresis phenomenon will not occur, and control of the film formingspeed during the film formation will be very easy.

[0045] When a metal target is used for forming a metal oxide film, thefilm forming speed or the sputtering voltage changes abruptly andnon-continuously in a hysteresis fashion due to a change of the oxygengas partial pressure before or after the change from an absorbing filmto a transparent film having the stoichiometrical composition, or beforeor after the change from a transparent film to an absorbing film.Accordingly, to obtain a transparent film constantly, it is necessary tointroduce oxygen gas substantially excessively relative to the metalatoms.

[0046] Whereas, the target of the present invention is composed of anoxide, and is slightly deficient in oxygen as compared with thestoichiometric composition. Accordingly, film formation of a transparentmetal oxide film can be carried out simply by supplementing the oxygenslightly deficient as compared with the stoichiometric composition.Besides, when the target of the present invention is employed, no changelike the above mentioned hysteresis phenomenon will take place, wherebythe amount of the oxygen gas to be supplied can be minimized to therequired minimum or reduced to a level close to the required minimum.Thus, deposition of excess oxygen atoms on the target surface, which isbelieved to cause deterioration of the film forming speed, can bereduced, and the film forming speed can be increased.

[0047] When a target is produced by a spraying method as in the presentinvention, the oxide powder is made in a molten state and then quenchedand solidified so that the sprayed material will be laminated on thesubstrate. At that time, crystal alignment will form in the sprayedmaterial, since there is a difference in the crystal growth rate ofcrystal faces. Namely, the face at which the surface density is low andthe growth rate is high, crystallizes quickly in the direction along thesubstrate, and crystal alignment will necessarily form, so that the faceat which the surface density is high and the growth rage is low, becomesthe sputtering surface.

[0048] On the other hand, it is believed that the higher the surfacedensity, the better the sputtering efficiency, and the higher thesputtering speed. Accordingly, this crystal alignment is believed to beone of the factors for the high film forming speed by the presentinvention. Further, sputtering speed is believed to be increased bynumerous defective layers formed in or between grains at the time ofquenching and solidification, which are susceptible to etching ascompared with normal tissues.

[0049] Further, with a sputtering target thus prepared, thermalconductivity from the ceramic layer to the substrate and further to thecathode electrode, is good, and the ceramic layer is firmly bonded tothe substrate. Accordingly, even when a high sputtering power is appliedto increase the film forming speed, cooling can sufficiently be carriedout, and peeling or cracking of the ceramic layer due to abrupt heatshock will not take place, and a large electric power per unit area canbe applied.

[0050] Further, even when an erosion zone of the ceramic layer becamethin, such a zone can readily be regenerated to the initial state byplasma spraying a ceramic powder of the same material to such a portionwhich became thin. Further, it is easy to provide a distribution inthickness of the ceramic layer depending upon any desired position, andit is thereby possible to control the thickness distribution of a thinfilm to be formed by providing a temperature distribution or adistribution in strength of the magnetic field at the target surface.

[0051] Further, when a cylindrical substrate is employed, the entiresurface will be the erosion zone of the target, whereby there is a meritthat the utilization efficiency of the target is high as compared withthe planer type.

BEST MODE FOR CARRYING OUT THE INVENTION EXAMPLE 1 TO 7

[0052] Commercially available Nb₂O₅ powder was filled in a hot pressingmold made of carbon, and hot pressing was carried out by maintaining itin an argon atmosphere for one hour at a temperature within a range offrom 1,100° C. to 1,400° C., as identified in Table 1. At that time, thehot pressing pressure was 50 kg/cm² The density and the resistivity ofthe sintered product thus obtained as a target material, were measured.

[0053] Then the obtained sintered body was pulverized in an agate mortarand then heated at 1100° C. in air, whereby the weight increase wasmeasured. Assuming that after such heating in air, the powder becamecompletely oxidized Nb₂O₅, the oxygen content of the sintered body afterthe hot pressing was calculated from the weight increase. The resultsare shown in Table 1. TABLE 1 Density Resistivity Oxygen Hot of the ofthe content pressing sintered sintered of the Example temperature bodybody sintered body No. (° C.) (g/cc) (Ωcm) (x in NbO_(x)) 1 1100 4.000.30 2.498 2 1150 4.13 0.21 2.495 3 1200 4.20 0.15 2.490 4 1250 4.300.14 2.465 5 1300 4.36 0.12 2.465 6 1350 4.40 0.12 2.465 7 1400 4.420.12 2.465

EXAMPLES 8 TO 11

[0054] The sintered body hot-pressed at 1200° C. in Example 3, wasmechanically processed to a size of 6 inch in diameter and 5 mm inthickness to obtain a target. The target was used as bonded by a metalbond to a backing plate made of copper.

[0055] This target was mounted on a magnetron DC sputtering apparatus,and film formation of a Nb₂O₅ film was carried out. The film formationwas carried out under such conditions that the applied power was DC 1kW, the back pressure was 1×10⁻⁵ Torr, and the sputtering pressure was2×10⁻³ Torr. As the sputtering gas, a gas having argon and oxygen mixedin various oxygen concentration, was used. The proportion of the oxygengas in the sputtering gas was from 10 to 40 volume %. If the oxygen islower than 10 volume %, the film will be an absorptive film, and inorder to obtain a transparent film, oxygen was required to be at least10 volume %.

[0056] As the substrate, a soda lime glass was used. No intentionalheating was applied to the substrate. The sputtering was carried out sothat the film thickness would be about 100 nm. During the sputtering,the electrical discharge was very stable, and film formation was carriedout under a stable condition even by DC sputtering. After the filmformation, the film thickness was measured by means of a feeler typefilm thickness measuring apparatus. Further, the refractive index of thefilm was measured by an ellipsometer. The wavelength of light employedat that time, was 633 nm. The film forming speed and the refractiveindex of the film are shown in Table 2. All of the films obtained weretransparent and showed no light absorption.

[0057] As is apparent from the results in Table 2 by using the target ofthe present invention, a transparent Nb₂O₅ film having a high refractiveindex was formed at a high speed.

COMPARATIVE EXAMPLES 1 TO 3

[0058] As Comparative Example 1, using a metal titanium target insteadof the target of Example 8, film formation by sputtering was carried outin the same manner. The proportion of oxygen gas in the sputtering gaswas 30 volume %. In the case of a titanium target, if oxygen is lowerthan 30 volume %, the film becomes an absorptive film, and in order toobtain a transparent film, oxygen was required to be at least 30 volume%. Accordingly, sputtering was carried out at an oxygen concentration of30 volume % which was the oxygen proportion at which a transparent filmcould be obtained and the film forming speed was highest.

[0059] Further, as Comparative Example 2, using a metal niobium targetinstead of the target of Example 8, film formation by sputtering wascarried out in the same manner. In the case of metal niobium target, thesputtering was carried out at an oxygen concentration of 30 volume % forthe same reason as mentioned above.

[0060] As Comparative Example 3, using a titanium monoxide (TiO) target(density: 4.90 g/cc, resistivity: 3. 0×10⁻⁴ Ωcm, oxygen content: 25.0 wt%), film formation by sputtering was carried out in the same manner. Inthe case of the TiO target, if oxygen is lower than 20%, the filmbecomes absorptive, and in order to obtain a transparent film, oxygenwas required to be at least 20%. Accordingly, sputtering was carried outby selecting a case of the oxygen proportion of 20% at which the filmformation speed was highest.

[0061] As is evident from the results in Table 2, the film formingspeeds for transparent Nb₂O₅ and TiO₂ in Comparative Examples 1 to 3,were inferior as compared with the cases in which targets of the presentinvention were used. TABLE 2 Oxygen Film content in the formingsputtering speed Refractive Target gas (volume %) (nm/min) index Example8 NbO_(x) 10 85 2.3 Example 9 NbO_(x) 20 70 2.3 Example 10 NbO_(x) 30 402.3 Example 11 NbO_(x) 40 30 2.3 Comparative Ti 30  6 2.4 Example 1Comparative Nb 30 13 2.3 Example 1 Comparative TiO 20  7 2.4 Example 1

EXAMPLES 12 TO 15

[0062] To commercially available Nb₂O₅ powder, an oxide of Cr, Ce, Al orSi was added in a proportion (oxide/oxide+Nb₂O₅) as identified in Table3 and mixed with the Nb₂O₅ powder in a ball mill. Such a powder mixturewas filled into a hot pressing mold made of carbon, and hot pressing wascarried out under the same conditions as in Example 3 to obtain asintered body. With respect to each sintered body, the density andresistivity were measured in the same manner as in Examples 1 to 7. Theresults are shown in Table 3. Such a sintered body was formed into atarget in the same manner as in Example 8, and film formation bysputtering was carried out, whereby the film forming speed was from 30to 90 nm/min, and the refractive index was 2.3.

[0063] Further, a part of each sintered body was subjected to aciddissolution or alkali fusion to obtain its aqueous solution, and thecomposition of the sintered body was analyzed by an ICP apparatuswhereby it was confirmed that the composition of the charged powdermixture and the composition of the sintered body were substantially inagreement. TABLE 3 Amount Density Resistivity of the of the of theExample additive sintered sintered No. Additive (wt %) body (g/cc) body(Ωcm) 12 Cr₂O₃ 20 4.52 0.45 13 CeO₂ 20 4.70 0.30 14 Al₂O₃  5 4.42 0.2015 SiO₂  5 4.21 0.20

EXAMPLE 16

[0064] By changing NbO_(x) in Example 3 to TaO_(x) (x=2.470), theoperation was carried out in the same manner as in Examples 8 to 11 andExamples 12 to 15, whereby similar good results were obtained.

[0065] Further, by changing TaO_(x) (x=2.470) to MoO_(x) (x=2.950),WO_(x) (x=2.955), ZrO_(x) (x=1.995, and 8 mol % of Y₂O₃ was added) andHfO_(x) (x=1.995), respectively, the operations were carried out in thesame manner, and similar good results were obtained.

EXAMPLE 17

[0066] High purity TiO₂ powder (average particle size: at most 10 μm)was mixed in a wet system for 3 hours in a ball mill using PVA binderand water as a medium, and the obtained slurry was granulated by meansof a spray drier to obtain a ceramic powder having a particle size offrom 20 to 100 μm.

[0067] Using a copper planer having a diameter of 6 inches as a targetmetal holder, the outer surface was roughened by sand blasting by meansof Al₂O₃ abrasive grains to obtain a roughened surface.

[0068] Then, an alloy powder of Ni—Al (weight ratio of 8:2) wasplasma-sprayed (using a metoko sprayer) under a reducing atmosphere toform an undercoat layer A having a thickness of 50 μm. This plasmaspraying under a reducing atmosphere was carried out using Al+H₂ gas asthe plasma gas at a flow rate of 42.5 l/min by applying a power of 35 kVat 700 A to instantaneously heat the alloy powder of Ni—Al by the Ar+H₂gas plasma of from 10,000 to 20,000° C. and to transport the alloypowder together with the gas onto the target metal holder to let itsolidify thereon. The coating film was formed by repeating an operationof moving the plasma spraying gun right and left and up and down.

[0069] Then, using a Ti metal powder, plasma spraying was carried out inthe same manner as above to form an undercoat layer B having a thicknessof 50 μm. Further, using the above mentioned ceramic powder, plasmaspraying was carried out under the same reducing atmosphere to form aceramic layer having a final thickness of 5 mm.

[0070] The ceramic layer of the obtained target was cut out from themetal substrate, and the density and the resistivity were measured.Further, the obtained ceramic layer was pulverized in an agate mortarand heated to 1100° C. in air, whereby the weight increase was measured.Assuming that after the heating in air, the powder became completelyoxidized TiO₂, the oxygen content of the ceramic layer was calculatedfrom the weight increase. The results are shown in Table 4. TABLE 4Density Resistivity (g/cc) (Ωcm) x in TiO_(x) Example 17 4.07 0.33 1.93

EXAMPLES 18 TO 21

[0071] The target of Example 17 was mounted on a magnetron sputteringapparatus, and film formation of a TiO₂ film was carried out by changingthe O₂ proportion in the sputtering gas as shown in Table 5. As thesputtering gas, Ar or a gas mixture of Ar and O₂, was used. Thesputtering was carried out under such conditions that the applied powerwas DC 1 kW, the back pressure was 1×10⁻⁵ Torr, and the sputteringpressure was 2×10⁻³ Torr. As the substrate, a soda lime glass was used,and no intentional heating was applied to the substrate. The sputteringwas carried out so that the film thickness would be about 100 nm. Duringthe sputtering, the discharge was very stable, and film formation wascarried out under a stabilized condition even by DC sputtering.

[0072] After the film formation, the refractive index of the film wasmeasured by an ellipsometer (the wavelength of light used was 633 nm).The film forming speed and the refractive index of the film are shown inTable 5. All of the obtained films were transparent and showed no lightabsorption. TABLE 5 Oxygen propotion Film- in the forming sputteringspeed Refractive Target gas (%) (nm/min) index Example 18 TiO_(x)  0 602.4 Example 19 TiO_(x) 10 30 2.4 Example 20 TiO_(x) 20 20 2.4 Example 21TiO_(x) 30 10 2.4

EXAMPLES 22 TO 25

[0073] Commercially available high purity TiO₂ powder and an oxidepowder as identified in Table 6 as an additive were mixed, so that theamount of the additive was as shown in Table 6. From such a powder, aceramic powder was prepared in the same manner as in Example 17, andplasma spraying was carried out in the same manner as in Example 17 toobtain a target having a ceramic layer having a thickness of 5 mm. Thedensity and resistivity of the obtained ceramic layer were measured, andresults are shown in Table 6.

[0074] A part of the ceramic layer constituting a target material wassubjected to acid dissolution or alkali fusion to obtain its aqueoussolution, and the composition of the sintered body was analyzed by anICP apparatus, whereby it was confirmed that the composition of thecharged powder mixture and the composition of the sintered body weresubstantially in agreement.

[0075] Further, with respect to the obtained target, film formation ofTiO₂ film was carried out in the same manner as in Examples 18 to 21.During the sputtering, the discharge was very stable, and film formationwas carried out under a stabilized condition even by DC sputtering. Thefilm-forming speed and the refractive index of the films were the sameas in Examples 18 to 21, and all of the obtained films were transparentand showed no light absorption. TABLE 6 Amount Density Resistivity ofthe of the of the Example additive sintered sintered No. Additive (wt %)body (g/cc) body (Ωcm) 22 Cr₂O₃ 20 4.30 0.62 23 CeO₂ 20 4.41 0.61 24Al₂O₃  5 4.09 0.22 25 SiO₂  5 4.01 0.15

EXAMPLE 26

[0076] In Example 17, instead of using the copper plate having adiameter of 6 inches as a target metal holder and roughening its outersurface by sand blasting by means of Al₂O₃ abrasive grains to aroughened surface, a copper cylindrical target holder having an innerdiameter of 50.5 mm, and outer diameter of 67.5 mm and a length of 406mm, was attached to a lathe, and its outer surface side was threaded andfurther surface roughened by sand blasting by means of Al₂O₃ abrasivegrains to form a roughened surface, and otherwise, in the same manner asin Example 1, a target was prepared. And, in the same manner as inExample 17, the density, the resistivity and the oxygen content of theceramic layer of the target were measured. The results were the same asin Example 17.

EXAMPLE 27

[0077] Film formation of a TiO₂ film was carried out in the same manneras in Example 18 to 21 except that the targets in Examples 18 to 21 wereused as the target of Example 26.

[0078] During the sputtering, the discharge was very stable, and filmformation was carried out under a stabilized condition even by DCsputtering. Further, all of the obtained films were transparent andshowed no light absorption.

[0079] The film-forming speed and the refractive indexes of the filmswere the same as in Examples 18 to 21.

EXAMPLE 28

[0080] A target was prepared in the same manner as in Example 17 exceptthat in Example 17, the obtained >ceramics powder was heat-treated at1,000° C. in an inert atmosphere to obtain a reduced powder, and theceramic layer was formed by water plasma spraying. The density and theresistivity of the ceramic layer of the obtained target were the same asin Example 17. Further, the oxygen content was examined in the samemanner as in Example 17 and found to be the same as in Example 17. Theceramic layer-forming time in a case where the ceramic layer of thetarget was 5 mm in thickness, is shown in Table 7 as compared withExample 17. TABLE 7 Ceramics layer-forming time (hrs) Example 17 2.5Example 28 0.3

INDUSTRIAL APPLICABILITY

[0081] By using the sputtering target of the present invention, atransparent film having a high refractive index can be formed at a highspeed by DC sputtering. Further, with the target of the presentinvention, the oxygen partial pressure of the sputtering atmosphere canbe reduced, thus providing a merit that abnormal discharge such asarcing can be reduced. Accordingly, by using the target of the presentinvention, a film having a high refractive index can be produced at ahigh speed and under a stabilized condition.

[0082] The target prepared by the process of the present invention isuniform, highly dense and strong against thermal shock. According to theprocess of the present invention, a target having an optional shape caneasily be produced without requiring conventional shaping, sintering,processing or bonding step.

[0083] Further, with the sputtering target of the present invention, thetarget can be regenerated by plasma spraying a spraying powder of freshtarget material having the same composition to a consumed portion afteruse, such being economically advantageous.

[0084] Further, in a case of a Ti type MO_(x), a photocatalytic functionis expected, and a film capable of imparting an antibacterial,antifouling or drip flowing property to a substrate surface of glass orother than glass such as plastic, can be formed at a high speed.

[0085] By using the sputtering target of the present invention, thecooling effect during the sputtering is high, whereby even if thesputtering power is increased, cracking or breakage of the target willnot take place, and film forming at a high speed can be carried outunder a stabilized condition at a low temperature. Thus, theproductivity of not only display elements or CRT but also large surfaceglasses for buildings or automobiles, can remarkably be improved.

[0086] Further, even on a substrate made of e.g. plastic which issusceptible to the influence of radiant heat from the target, high speedfilm forming can be carried out without damaging the substrate.

1. A sputtering target comprising a substrate and a target materialformed on the substrate, wherein the target material-comprises a metaloxide of the chemical formula MO_(x) as the main component, whereinMO_(x) is a metal oxide which is deficient in oxygen as compared withthe stoichiometric composition, and M is at least one metal selectedfrom the group consisting of Ti, Nb, Ta, Mo, W, Zr and Hf.
 2. Thesputtering target according to claim 1 , wherein in said MO_(x), M is Nband/or Ta, and x is within a range of 2<x<2.5.
 3. The sputtering targetaccording to claim 1 , wherein in said MO_(x), M is Mo and/or W, and xis within a range from 2<x<3.
 4. The sputtering target according toclaim 1 , wherein in said MO_(x), M is at least one metal selected formthe group consisting of Ti, Zr and Hf, and x is within a range of 1<x<2.5. The sputtering target according to any one of claims 1 to 4 , whereinthe target material has a resistivity of at most 10 Ωcm at roomtemperature.
 6. A process for producing a sputtering target, whichcomprises forming an undercoat made of a metal or alloy on a substrate,and forming a ceramic layer as a target material on the undercoat,wherein the ceramic layer as a target material is formed by plasmaspraying wherein a ceramic powder which is made in a semi-molten statein a high temperature plasma gas in a reducing atmosphere, istransported and deposited onto the undercoat by the plasma gas, and, asthe target material, a target material comprising a metal oxide of thechemical formula MO_(x) as the main component, is used, wherein MO_(x)is a metal oxide which is deficient in oxygen as compared with thestoichiometric composition, and M is at least one metal selected fromthe group consisting of Ti, Nb, Ta, Mo, W, Zr and Hf.
 7. The process forproducing a sputtering target according to claim 6 , wherein, as theundercoat, a layer having a thermal expansion coefficient intermediatebetween the thermal expansion coefficient of the ceramic layer and thethermal expansion coefficient of the substrate, and/or a layer having athermal expansion coefficient close to the thermal expansion coefficientof the ceramic layer, is used.
 8. The process for producing a sputteringtarget according to claim 6 , wherein the plasma spraying is waterplasma spraying.
 9. The process for producing a sputtering targetaccording to claim 6 , wherein a cylindrical substrate is used as thesubstrate.
 10. The process for producing a sputtering target accordingto claim 6 , wherein a surface-roughened substrate is used as thesubstrate.
 11. A method for forming a film having a high refractiveindex by sputtering, wherein, as a sputtering target, the sputteringtarget as defined in any one of claims 1 to 5 is used.